Add support for pixi lock files

[matthewfeickert]

conda-lock itself hauls in... a lot of dependencies, so may not be a good candidate for the "base coat" environment. micromamba, already present, is certainly up to the task of consuming both formats... though pixi very well might end up "winning" for this use case.

Originally posted by @bollwyvl in #1312 (comment)

This was already touched upon in Issue #1312 a bit (in the above), and now also with conda/conda-lock#615 being open, but it would be quite nice if pixi lock files could be supported formally by repo2docker. The ability to do cross-platform solves in parallel already brings the accessibility barrier for projects down quite a bit, and then being able to reuse that environment solve for repo2docker is very attractive from a user perspective.

I appreciate that supporting a new standard is a lot fo work, and @bollwyvl has already shown in Issue #1312 that these sorts of changes are hard. I'm asking as I think it would be very cool!